Test dataset for python Lagarangian particle package seaduck

Test datasets for seaduck. Including:

1. C-grid lat-lon surface velocity derived from AVISO sea-surface height product.

2. 3-Month subset from monthly mean ECCOv4r4 product.

3. Two small datasets from MITgcm runs with rectilinear and curvilinear grid, inherited from test dataset of python package oceanspy

4. One snapshot from the ASTE reanalysis product.